After that, QKD has been extensively studied [1]}, [2]}, [3]}, [4]} and grown to a matured technology in a real-world application today. Quite a few metropolitan fiber network has been built all over the world [5]}, [6]}, [7]}, [8]}, [9]}, [10]}. Similar as the optical fiber communication (OFC), the quantum state will also be attenuated exponentially with the transmission distance. For OFC, optical amplifier is used to rely the optical signal every 80 km [11]} to build up a long haul fiber network. The amplifier for single unknown quantum state, however doesn't exist because the amplification of quantum signal can also be seen as a type of clone and any cloning of unknown quantum states will bring fatal errors [12]}. Therefore, a simple optical amplifier can't be used for long haul fiber QKD. So far the longest reported field test of QKD is around 90 km [13]}.
